Unlike a conventional cathode ray tube, a liquid crystal display (LCD) that is recently popular as a general display means or a monitor consumes very low electric power and is thin and light since it does not employ the electronic gun. So, in these days, the LCD has been rapidly developed and widely used.
Since the LCD cannot emit light by itself, it employs a method controlling light amount by reflecting or transmitting light. The light source employed in the transmission type LCD that is widely used is called a backlight unit. The performance of the LCD depends seriously on that of the backlight unit.
FIGS. 1 and 2 are diagrams illustrating structure of a backlight unit employed in the LCD. The backlight unit includes a fluorescent lamp 1, a reflecting plate 2, a light guide plate 3, a diffusion sheet 4, a first prism sheet 5, a second prism sheet 6 and a protection sheet 7. An LCD panel 8 is mounted on the top of the backlight unit.
As shown in FIG. 2, if a voltage is applied to the fluorescent lamp 1, residual electrons in the fluorescent lamp 1 moves to anode. When the residual electrons moving to the anode of the fluorescent lamp 1 collide with molecules of argon (Ar), argon is excited and thereby cation increases. The increased cation collides with cathode of the fluorescent lamp 1 and secondary electrons are emitted from the cathode of the fluorescent lamp 1.
The emitted secondary electrons flow inside a tube of the fluorescent lamp 1 and starts discharge. The electrons flowing due to the discharge collide with mercury vapor. The mercury vapor is ionized to emit ultraviolet ray and visible light. The emitted ultraviolet ray excites fluorescent substance coated on the inner surface of the tube of the fluorescent lamp 1 to emit visible light.
The emitted visible light travels along the light guide plate 3 that has a slant lower surface. On the lower surface of the light guide plate 3, a variety of patterns such as a fine dot pattern are printed in order to change the traveling direction of the visible light toward the LCD panel 8 so that light loss is reduced and the light is guided to an upper surface of the light guide plate 3.
Here, the light transmitting through the upper surface of the light guide plate 3 includes not only beams emitted perpendicular to the upper surface but also beams emitted slant in various angles.
Between the light guide plate 3 and the first prism sheet 5, a diffusion sheet 4 is placed to diffuse the light that is incident from the light guide plate 3, thereby preventing the light from being locally concentrated and also functioning to change the direction of the light traveling toward the first prism sheet 5 and to thus reduce a slant angle with respect to the first prism sheet 5.
The first prism sheet 5 and the second prism sheet 6 are provided with triangular prisms formed in a predetermined arrangement on the upper surfaces thereof. The first prism arrangement of the first prism sheet 5 and the second prism arrangement of the first prism sheet 6 are crossed with each other by a predetermined angle.
The first prism sheet 5 and the second prism sheet 6 condense the light diffused from the diffusion sheet 4 in the direction perpendicular to the LCD panel 8. Accordingly, the orthogonal incidence characteristic of the light passing through the first prism sheet 5 and the second prism sheet 6 to the protection sheet 7 can be obtained completely.
Therefore, the light passing through the first prism sheet 5 and the second prism sheet 6 travels almost perpendicularly with respect to the first prism sheet 5 and the second prism sheet 6, so that the brightness distribution on the protection sheet 7 can be obtained uniformly.
According to the prism sheet adapted to the conventional backlight unit, the peaks of the prism sheet are maintained at the same height along the length direction of the stripe shape, so that the optical coupling is made between the peak portion of the first prism sheet 5 positioned at a lower portion and the flat surface of the second prism sheet 6 positioned at an upper portion if the first prism sheet and the second prism sheet are used in fold. So, the moiré phenomenon that a pattern of lines is visibly shown is caused and the quality of image displayed on a screen deteriorates.
The prism sheet structure adapted to a backlight unit is disclosed as a flexible thin film in Korean patent laid-open publication No. 1987-0005258.
FIGS. 3A and 3B illustrate a flexible film employed as a prism sheet of the backlight unit. This flexible thin film has a structural surface 12 on one side thereof and a flat surface 14 on the other side thereof and is made of transparent polymer material.
The structural surface 12 of the film includes a linear arrangement of small right-angled isosceles triangular prisms arranged in parallel and is configured to have a plurality of peaks 17 and grooves 18 that are as long as the length of the film 10 as shown in FIGS. 4 and 5. When the film is positioned on the flat surface, the perpendicular surfaces 20 slant by an angle (α) of about 45° with respect to the adjacent flat surface 14.
The incident light that strikes the structural surface 12 or the flat surface 14 in a predetermined angle is fully reflected from the other surface as shown in FIG. 4. If the light refracted by a first surface strikes a second surface by an angle larger than the critical angle with respect to the normal line, the light is fully reflected. The critical angle in air is defined to be arc sine of reciprocity of index of refraction of the material. As shown in FIG. 5, a great deal of the incident light striking the surface 12 or the surface 14 by an angle larger than the angle range is transmitted and the remainder is reflected. In both cases, the optical absorption due to the material is negligible.
The light ‘A’ that is incident into each of the flat surfaces 14 formed along a normal line of the flat surface 14 of the flexible thin film is refracted, and fully reflected at the structural surface 12 of the flexible thin film as shown in FIG. 3. Both the light ‘A’ and the normal line ‘N’ lie on the plane perpendicular to the direction P in which the prisms 16 of the structural surface 12 are arranged linearly. The light ‘A’ is fully reflected and projected as a reflection light ‘A′’ located on the same plane. Similarly, another light ‘B’ applied to the flat surface 14 on the plane not perpendicular to the direction P is exemplarily shown. The incident light B is reflected inside, and projected as the light ‘B’ on another plane defined by the incident light B and the prism direction P.
However, when the flexible thin film is employed as the prism sheet of a backlight unit, the peaks of the prism sheet are maintained at the same height along the length direction of the stripe shape. So, if the first prism sheet and the second prism sheet are used in fold, the optical coupling is made between the peak portion of the first prism sheet positioned at a lower portion and the flat surface of the second prism sheet positioned at an upper portion. So, the moire phenomenon that a pattern of lines is visibly shown caused and the quality of image displayed on a screen deteriorates.
Accordingly, the present invention is directed to a prism sheet of a backlight unit for an LCD that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a prism sheet effectively applicable to a backlight unit.
Another object of the present invention is to reduce an optical coupling occurring in case peaks of prism sheets of a backlight unit are maintained at the same height along the length direction of the stripe shape and a first prism sheet and a second prism sheet are used in fold, by changing the structure of the prism sheet.
A further object of the present invention is to reduce moire phenomenon by minimizing an optical coupling caused between prism sheets when the prism sheets are arranged in fold so as to constitute the backlight unit.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention is characterized by a prism sheet having a structural surface on one side thereof and a flat surface opposing the structural surface on another side thereof, the structural surface including a linear arrangement of right-angled isosceles triangular prisms arranged in parallel and configured to form a plurality of peaks and valleys, each of the prisms having perpendicular surfaces that slant by an angle of approximately 45° with respect to the flat surface,
wherein the structural surface is configured to have non-planar peaks with a maximum height and a minimum height along a length direction of the peak, and a curved layer having the same cycle as a cycle of height variation of the peak is formed at a boundary surface between the structural surface and the flat surface so as to maintain the right-angled isosceles triangular prisms formed due to a difference between the highest point and the lowest point of each of the peaks to have a predetermined size so that a distance between the valleys is uniform along the length direction.
When the structural surface of the prism sheet is changed and used as a prism sheet of a backlight unit, the optical coupling caused between the structural surfaces of the corresponding prism sheets is minimized and the moiré patterns can be suppressed or removed, so that highly reliable prism sheets adapted to the backlight unit can be obtained.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.